Digitally tunable SIW MMIC resonators for E band and mmWave

This work presents the design, modeling, and experimental validation of both digitally and analog tunable gallium arsenide (GaAs) monolithic microwave integrated circuit (MMIC) substrate-integrated waveguide (SIW) resonators for E-band applications. Initially, series- and parallel-equivalent circuit models were investigated for a fixed SIW cavity to establish a baseline understanding of its resonant behavior. These models were subsequently extended to shunt RLC-based representations with coupling-dependent perturbation elements to accurately capture the interactions between pHEMT devices, a varactor, and the SIW cavity in the proposed tunable architectures. The first tunable resonator concept employs pHEMT cold-FET switches, implementing a fully digitally tunable resonator, while the second concept uses both the digital cold-FET switches and a varactor to provide hybrid (digital-plus-analog) frequency tuning. Two reconfigurable implementations were developed: a six-switch topology providing fine frequency resolution with a simulated tuning range of 580 MHz and a measured span of approximately 200 MHz while maintaining high unloaded Q factor ( Qu ) and a four-switch-plus-varactor configuration achieving a simulated tuning range of approximately 1.13 GHz and a measured tuning of circa 600 MHz, with Qu consistently above 80. These results demonstrate that compact, on-chip SIW resonators with embedded semiconductor tuning elements can achieve good frequency agility with minimal quality factor degradation, highlighting their suitability for digitally and hybrid-tunable reconfigurable oscillators and adaptive E-band front-end systems.